Field of the Invention
The present invention concerns a sialon of wurtzite structure excellent in hardness and oxidation resistance, as well as heat conductivity, which is usable effectively for dies and machinery parts, as well as a synthesizing method therefor.
Crystalline sialon has been studied earnestly mainly in England and Japan from since it was pronounced by Professor Jack in England in 1976.
Sialon is a generic term for crystalline materials containing Si, Al, O and N for which each of .beta.', .alpha.', X and O ' phases and a polytypoid phase (15R, 12H, 21R, 27R, etc.) have been known. Among them, .beta.' sialon has the same structure as .beta.-Si.sub.3 N.sub.4 (phenacite structure), and a composition of Si.sub.6-z Al.sub.z 0.sub.z N.sub.8-z (z=0.5-4.2). The mechanical characteristics is different depending on the value z in which the mechanical characteristic is higher for the lower z value and the highest mechanical property with 18 GPa of hardness and 900 MPa of bending strength is attained at z=0.5, and the bending strength does not deteriorate as high as the vicinity of 1000.degree. C. Furthermore, the crystalline sialon has such excellent high temperature characteristic as it is stable to molten metal such as Al.
Therefore, .beta.' sialon has been produced by a sintering process by a number of manufacturers including those in this country.
However, since .beta.' sialon marketed at present is obtained by adding a sintering aid such as MgO or Y.sub.2 0.sub.3 to a starting powdery mixture of Si.sub.3 N.sub.4 SiO.sub.2 --AlN system or Si.sub.3 N.sub.4 --Al.sub.2 O.sub.3 --AlN system and sintering them at 1700.degree.-1800.degree. C. in a nitrogen gas stream, sintering products include defects such as pores or grain boundary impurity phase, to vary various characteristics such as strength and improvement therefor is expected.
In view of the above, if crystalline sialon can be synthesized by a gas phase process, the sintering aid is no more basically necessary and it can be expected to obtain homogeneous products at high density and purity. Further, if crystalline sialon can be synthesized on substrates made of various materials by a gas phase process, additive value of sialon as the material can be improved remarkably.
Si--Al--O--N system materials have heretofore been synthesized by a sputtering process using crystalline sialon prepared by a sintering process as a target. However, the resultant Si--Al--O--N system material obtained by the sputtering process is amorphous, the hardness of which is about 14 GPa and the deposition rate is as low as about 15 nm/min.
Other than the sputtering process, Si--Al--O--N system materials have been synthesized, in few cases, by hot CVD process and reactive PVD process. Landingham, et al of California University tried to synthesis sialon on a graphite substrate by using a AlCl.sub.3 --SiCl.sub.4 --NH.sub.3 gas system, but it only produced an amorphous Si--Al--O--N system material which was not crystalline sialon. Further, Chin, et al of General Atomic Co. tried to synthesize a Si--Al--O--N system material by a reactive ion plating process, but the resultant product was also amorphous and, therefor, it has not yet been realized till at presence to synthesize crystalline sialon by the gas phase process.